I have a 2005 nissan sentra that the heater works sometimes and sometimes not. I replaced the thermostat and still having the problem also the tempeture guage sometimes fluxuates .

could have a bad blend that controls both heat and ac

Might try a reverse flush of the cooling system. Might reverse flush the heater core too.

you may have a problem with heater box. your air/mix door may have an issue. feel the knobs on the control unit. if the cold/heat know feels kinda funny from the other knobs, then thare is a problem with heater box/cables??

Is it possible that you have a temp door problem in the dash. Maybe the cable from the temp control came off the arm for the door.

It sounds like your fans are not operating properly. When u accelerate cool air is forced through the radiator & condenser which can cause that problem , u can change the thermo. for maintenance anyway. Use fresh 50/50 coolant mixture. Ck those fans

Are you sure there is not a air pocket in the engine after changing water pump,
there is only 1 thermostat

i have a 2005 1.8 nissan sentra.i rplaced the radiator becuause it was leaking.so when i did i took it for a test drive and the needle went to hot then back down to middle ways between the C & H BACK IN FORTH.so i replaced the thermostat. now as far as i can tell it is staying in the middle of C & H. i can't feel no water though cerulating through the top or bottom hose of the radiator. so now that i told you what i done so far my question is???? ( my heater is not working in the car it is blowing out cold air & i left the car runnng for about 45 min. even took it up and down the road.still cold air.what do you think i need to do?????????? )

check cooling fan sensor and cooling fan motor

try to replace the thermostat and flush the coolent system and dont forget to bleed the coolent system when u done

Hope this helps, please don't forget to rate me.
thanx

My 2005 Nissan Sentra 1.8s with 55,000 mi. blows cold air from the heater. Coolant not circulating in engine. Lower hose stays cool to touch. Both heater hoses cold. Changed thermostat - didn't fix it. Coolant not leaking or dirty. None in oil - no oil in radiator. Changed worn water pump belt - pump spins freely with no noise or resistance. Cooling fans come on after warm-up. Temperature gauge stays in normal range then jumps to hot and back again. Coolant good to 10 below - didn't freeze here yet. No other problems or repairs to car. Afraid to drive and make worse. Help!